The invention relates to an electrostimulator having an output connection to a working electrode and a test generator having an output connected to the output connection for generating either a pulse-shaped or periodically changeable test signal and feeding the signal to the output connection.
Implantable pacemakers have been used for some time to treat cardiac disfunctions, as is known, and in particular bradycardia conditions. These pacemakers transmit electrical stimulation pulses to the heart via an endocardially arranged stimulation electrode if the heart stops beating or does not beat sufficiently fast.
Owing to the fact that each stimulation pulse leads to a partial discharge of the pacemaker battery, efforts are made to lower the amplitude of the stimulation pulses as much as possible to increase the battery service life, wherein it should be taken into consideration that the heart will no longer react with a contraction to a stimulation with an amplitude below a specified threshold value, also referred to as stimulus threshold value.
It is therefore also known to conduct a so-called stimulus threshold value test to determine the stimulus threshold value of the heart individually for each pacemaker carrier and to be able to program the stimulation pulse amplitude accordingly. For this, the pacemaker emits successive stimulation pulses with a decreasing amplitude, wherein it is respectively determined whether the heart reacts with a contraction to the preceding stimulation pulse by evaluating an extracorporeal recorded electrocardiogram (ECG). The stimulus threshold value for the heart in that case approximates the amplitude at which the heart is barely stimulated by the stimulation pulse.
However, one problem with this is that a change in the stimulus threshold value, e.g. due to changes in the chronic stimulus threshold, is not detected during the normal pacemaker operation, which can lead either to a stimulation with unnecessarily high amplitudes or--considerably worse--to an unsuccessful stimulation.
That is why in recent years pacemakers have become known which determine automatically whether the heart is successfully stimulated by a stimulation pulse and which accordingly optimize the amplitude for the stimulation pulses. For this, the pacemaker measures the so-called evoked potential by means of the pacemaker electrode, in each case immediately following a stimulation pulse, which evoked potential causes the cardiac muscle contraction and represents the response to the preceding stimulation pulse. The problem is that the electrode system which encloses two metal electrolytic boundary surfaces is electrically charged with a stimulation pulse, owing to its capacitive properties, so that the evoked potentials can be concealed by the electrical after-effects of a stimulation pulse (artifacts on both boundary layer capacities). For that reason, this concept is only used in connection with high-capacity electrodes which, owing to their high capacity, are charged only to a relatively low voltage by a stimulation pulse, which does not interfere with the detection of the evoked potential.
Until now, suitable electrodes were selected on the basis of an extracorporeal measurement of the electrode capacity by means of separate measuring instruments, resulting in higher implantation expenditure and the disadvantage that a post-operative change in the electrode capacity is not detected by the pacemaker. Problems furthermore had to be expected with the new implantation of a pacemaker and continued use of the previously implanted electrode.